headway

noun

Etymology: Partly from Middle English hauedwei, from Old English hēafodweġ (“head-road, main-road”); partly as a shortening of ahead-way, the source of the nautical sense. By surface analysis, head + way.

1. A movement ahead or forward.
2. Forward motion, or its rate.
3. The interval of time or distance between the fronts of two vehicles (e.g. buses) moving in succession in the same direction, especially along the same pre-determined route.

   “The whole of the main lines to be electrified were being equipped with four-aspect colour-light signals, automatically operated, where appropriate, and spaced to give a 5min headway throughout.”

   “Sixty minutes divided by the headway gives a potential figure for 'trains per hour' - the simplest possible and most simplistic definition of capacity, termed line capacity.”

4. Progress toward a goal.
5. The clearance beneath an object, such as an arch, ceiling or bridge; headroom.

   “So as not to impede navigation the two principal spans of the bridge were designed to provide a clear headway of 157 ft. above high water for a distance of 500 ft.; [...].”

6. A crossheading.